BEIJING -- An expedition searching for a rare Yangtze River dolphin ended Wednesday without a single sighting and with the team's leader saying one of the world's oldest species was effectively extinct.

The white dolphin known as baiji, shy and nearly blind, dates back some 20 million years. Its disappearance is believed to be the first time in a half-century, since hunting killed off the Caribbean monk seal, that a large aquatic mammal has been driven to extinction.

A few baiji may still exist in their native Yangtze habitat in eastern China but not in sufficient numbers to breed and ward off extinction, said August Pfluger, the Swiss co-leader of the joint Chinese-foreign expedition.

"We have to accept the fact, that the Baiji is functionally extinct. We lost the race," Pfluger said in a statement released by the expedition. "It is a tragedy, a loss not only for China, but for the entire world. We are all incredibly sad."

Overfishing and shipping traffic, whose engines interfere with the sonar the baiji uses to navigate and feed, are likely the main reasons for the mammal's declining numbers, Pfluger said. Though the Yangtze is polluted, water samples taken by the expedition every 30 miles did not show high concentrations of toxic substances, the statement said.

For nearly six weeks, Pfluger's team of 30 scientists scoured a 1,000-mile heavily trafficked stretch of the Yangtze, where the baiji once thrived. The expedition's two boats, equipped with high-tech binoculars and underwater microphones, trailed each other an hour apart without radio contact so that a sighting by one vessel would not prejudice the other.

Around 400 baiji were believed to be living in the Yangtze in the 1980s. The last full-fledged search, in 1997, yielded 13 confirmed sightings, and a fisherman claimed to have seen a baiji in 2004, Pfluger said in an earlier interview.

At least 20 to 25 baiji would now be needed to give the species a chance to survive, the group's statement said, citing Wang Ding, a hydrobiologist and China's foremost campaigner for the baiji.